I haven't built an amp with them per se, but I have two little amp modules that came out of an old high end television. Each has a 12AX7 voltamp/phase splitter and two 6gk6 outputs. They sounded very nice actually. I also rewired them as triode, dropped the overall feedback and tried them that way for a while, also good. Have been meaning to incorporate them into a little boombox stereo, but my to do list is just to darn long at the moment. I believe they are identical to the 6BQ5 save the pinout difference. Don't know if that helped, but in my opinion they are worth playing with.